Incoming is not relevant for form sending email.
For sending with the builtin, I believe you need either port 587, 2525 or 25, as encryption is set to/hardcoded to STARTTLS. For 465, you’d need to change the script to use SMTPS (unless newer Blocs automates this).
It’s a question of what your email provider supports, and about what your webhost allows on outbound firewall.